The present invention relates to a process for recovering oil and sulfonates from filtration panels.
Oil-soluble sulfonates of earth-alkaline metals are widely used as detergent additives for lubricating oils. They are obtained by sulfonating hydrocarbon mixtures with a high content of aromatic alkyl hydrocarbons, characterized by a long lateral alkyl chain bound to the aromatic ring.
The hydrocarbon mixture can be of synthetic origin (and therefore consisting of aromatic alkyl hydrocarbons produced by the alkylation of benzene with a polymer of propylene) or it can derive from heavy refinery distillates characterized by a high content of aromatic alkyl hydrocarbon. Typical heavy refinery distillates suitable for the purpose have a weight average molecular weight of about 480.
As far as synthetic products are concerned, these can sometimes have a lower molecular weight, but the weight average molecular weight is higher than 400.
The sulfonic acids produced in the sulfonation process are then converted into sulfonates of earth-alkaline metals.
The conversion of sulfonic acids to sulfonates of earth-alkaline metals can be effected by direct neutralization with a base compound of earth-alkaline metal, such as hydroxide, oxide or carbonate. This conversion can also be carried out by the neutralization of sulfonic acids with a hydroxide of an alkaline metal; the alkaline metal sulfonate thus formed is then converted to sulfonate of earth-alkaline metal by metathesis with a compound of earth-alkaline metal such as hydroxide, oxide, carbonate, chloride.
The sulfonates can be neutral salts obtained by the reaction of sulfonic acids with the stoichiometric quantity required for neutralizing acids, or they can be overbased. Overbased sulfonates can be prepared by means of contact of sulfonic acids (or salts of the alkaline metals of the above acids) with an excess, with respect to the stoichiometric value, of a base compound of earth-alkaline metal. The process consists in mixing the sulfonic acid (or its metal alkaline salt) with an excess of base earth-alkaline compound, usually in the presence of a hydrocarbon solvent, and introducing gaseous carbon dioxide into the mixture. An alcohol is commonly used to promote the production of over-based sulfonate.
The process to which the present invention refers is well known to experts in the field, see for example xe2x80x9cProcxc3xa9dxc3xa9 de suralcalinisation d""additifs dxc3xa9tergents: xe2x80x9cRÔle des Promoteurs et determination du rxc3xa9gime de la rxc3xa9action de surbasagexe2x80x9d R. Gallo and F Jacquet, Revue de l""Institut Francais du Petrol, Vol. 476, Nr. 2, Mars-Avril 1991.
The final step of the production process of neutral or overbased sulfonates is the separation of the desired products from the by-products, particularly non-reacted compounds of earth-alkaline metals and sediments. It is important for the detergent additive to have a colloidal structure with sufficiently small particle dimensions (usually  less than 0.1 micron) so as to be transparent to visible light (see U.S. Pat. No. 3,155,617) and easily filterable through the usual filtering devices. In fact, if particles of non-reacted alkaline or earth-alkaline material are present in the additive, or if almost insoluble large dimensional colloidal structures have been created in the synthesis phase, without a suitable liquid-solid separation step, abrasions and decantation can occur when using the lubricating oil.
The purification step can be carried out before or after the removal of the reaction solvents.
The end additive proves to consist of a mixture of sulfonate (overbased) formed and of the lubricating base added during the process to lower the viscosity. In the case of overbased additives, the basicity reserve, measured by TBN according to the procedure described in WO 97/46443, depends not only on the ratios between the reagents but also on the dilution with the lubricating base.
The purification step of the additives based on sulfonates usually consists of a filtration with aids. The material to be removed is normally made up of the non-reacted base oxide or hydroxide, impurities of the starting oxide or hydroxide consisting of carbonates of the metal itself or oxides or hydroxides of other metals, sulfonates deriving from the neutralization of the sulfuric acid optionally present in sulfonic acids, crystalline carbonates formed during the reaction, unstable micelles of sulfonate (sediment) formed during the reaction which prove to be non-oil-soluble owing to the considerable dimensions of the carbonate nucleus and/or of an insufficient coating thereof on the part of the surface-active agent based on sulfonate.
The content of xe2x80x9csedimentxe2x80x9d in the product to be filtered varies according to the synthesis process, the raw materials used and degree of overbasicity required, and it is mainly this impurity which determines the filtration rate and quantity of filter aid to be used U.S. Pat. No. 3,537,996 identifies xe2x80x9cconsiderable quantities of solids prevalently consisting of colloidal particles of calcium carbonate having large dimensions, not dispersible in the oily medium (sediment)xe2x80x9d, as main impurities to be eliminated by filtration. U.S. Pat. No. 3,155,616 describes how some processes for producing colloidal dispersions of calcium carbonate which use sulfonates, are made difficult by the presence of materials which tend to deteriorate upon contact with the hot surfaces of the process equipment, forming agglomerates of large dimensional particles which cannot be tolerated in the end lubricating product and which can only be removed with great difficulty due to their substantial non-filterability.
In solid-liquid purification by filtration, U.S. Pat. No. 3,537,996 identifies amorphous diatomaceous siliceous earth as a suitable aid, both for forming the precoat on the filtration surface and to be added to the product to be purified, and indicates a quantity of 1 to 15% in the product to be purified as appropriate. U.S. Pat. No. 3,155,617 also specifies diatomaceous earth as suitable aid and lists known commercial types, such as Filter-Cel, Hy-Flo, Super Cel and Dicalite.
During the filtration process with aids, a filtration panel is produced which contains impurities present in the reaction product, the filter aid and the detergent additive. In particular the content of additive in the panel is very high and is comparable to that of the aid.
The filtration panels obtained in the processes described above should be discharged as undesired by-products of the sulfonation process. This causes a loss in oil and sulfonate. In addition, a more important factor is that these filtration panels are classified as potentially dangerous materials, with obvious disposal problems.
The necessity is therefore felt for improving the purification process of sulfonate, thus avoiding disposal of the above panels.
To solve this problem, U.S. Pat. No. 4,614,597 treats the filtration panel with acid selected from phosphoric acid and sulfuric acid until reaching a pH ranging from 2 to 7, and maintaining the mixture thus obtained at a temperature of over 150xc2x0 F. The hot mixture is subsequently left to rest until separation of the two phases, the upper phase containing oil and the sulfonate, the lower phase containing the filter aid and the acid.
U.S. Pat. No. 4,501,670 on the other hand, describes a process in which the filtration panels are mixed with hot aqueous solutions of materials selected from hydroxides of alkaline metals, salts of alkaline metals of acids having ionization constants of less than 1.5xc2x710xe2x88x924 and relative mixtures. After the separation has been completed at temperatures of over 160xc2x0 F., two phases are separated which are recovered.
Both processes of the prior art reduce or eliminate the dangerousness of the panel, but have the disadvantage of requiring subsequent washing and neutralization phases. In addition, these phases do not allow the starting additive to be recovered, but rather an intermediate to be recycled, for example sulfonic acid to be recycled to the synthesis process of sulfonates, in the case of U.S. Pat. No. 4,614,597, or sodium sulfonate to he reconverted to calcium sulfonate in the case of example 1 of U.S. Pat. No. 4,501,670. This makes the process less economic.
In addition, the separation of the oily phase, containing the intermediate recovered, from the aqueous phase, containing the filter aid, is carried out by decanting, with the possibility of forming stable emulsions which can negatively influence the operating times.
A process has now been found which overcomes the drawbacks described above.
In accordance with this, the present invention relates to a process for treating filtration panels obtained by the filtration of oils containing sulfonates of earth-alkaline metal, the above filtration being carried out in the presence of filter aids essentially consisting of siliceous material, preferably diatomaceous earth, which comprises:
(a) dilution of the filtration panel with a C4-C8 aliphatic hydrocarbon, preferably hexane, and addition of another filter aid, thus obtaining a suspension of the panel and of the filter aid in the aliphatic hydrocarbon;
(b) filtration of the suspension of step (a), thus obtaining a solid essentially consisting of filter aid with a minimum content of sulfonate and a turbid C4-C8 hydrocarbon solution (i);
(c) centrifugation of the turbid hydrocarbon solution (i), thus obtaining a solid and a purified hydrocarbon solution (ii);
(d) treatment of the purified hydrocarbon solution (ii) with means suitable for eliminating the C4-C8 hydrocarbons, thus recovering the sulfonate of earth-alkaline metal.
The filtration panel, subsequently subjected to the process of the present invention, essentially consists of about 50% of oil, sulfonate and reaction residues, and the remaining 50% of the filter aid.
In step (a) of the process of the present invention, the above panel is treated with a quantity of hydrocarbons selected from those ranging from C4 to C8, preferably n-hexane, and with a filter aid of the silicon type, preferably diatomaceous earth. The weight ratio between C4-C8 hydrocarbons and panel is not determinant for the process of the present invention. In the preferred embodiment however, the weight ratio panel/C4-C8 hydrocarbons ranges from 1/0.5 to 1/20, more preferably from 1/1 to 1/10, even more preferably from 1/2 to 1/5.
As far as the quantity of filter aid added is concerned, the weight ratio between the panel and filter aid ranges from 1/0.2 to 1/5, more preferably from 1/0.4 to 1/3, even more preferably from 1/0.6 to 1/1.5.
The three components (panel, hydrocarbons, filter aid) are preferably mixed at a temperature ranging from 15xc2x0 C. to 40xc2x0 C., for a time sufficient to obtain good contact between the various components. Using appropriate stirring devices, a few minutes are usually sufficient.
Step (b) consists in the filtration of the suspension of step (a). Filtration devices well known to experts in the field can be used for the purpose. The use of filter presses is particularly effective. The filtration produces a turbid hydrocarbon filtrate (i) and the filter aid which proves to have a minimum content of sulfonate.
Step (c) of the process of the present invention consists in the centrifugation of the turbid hydrocarbon solution (i), the centrifugation being carried out using the usual centrifuges commercially available (laboratory, or pilot plant or industrial). When operating on an industrial scale, a centrifuge of the disc-stack type in the self-cleaning version, with an opening bowl for the discharge of the solids accumulated, or a centrifuge of the disc-stack type with continuous discharge of the solids, can be used.
This step produces a solid and a purified hydrocarbon solution (ii).
The above solid mostly consists of organic material with a micelle structure not oil-soluble in a paraffinic environment (sediment). Part of the solid on the other hand consists of inorganic material of various origins, which is very fine and not withheld in the filtration step (b).
As far as the inorganic part is concerned, this usually consists (a) partly of very fine inorganic material not withheld by the filtration and deriving from filter aids (for example silica and titanium dioxide); (b) partly of non-converted reagents (for example calcium hydroxide); (c) partly of impurities of the starting raw materials; (d) partly of non-colloidal material formed during the synthesis (for example calcium carbonate).
The final step of this process (step d) consists in eliminating the hydrocarbon solvent from the purified hydrocarbon solution (ii). This step can be carried out according to the known techniques for example distillation at atmospheric pressure or at reduced pressure. As will be shown further on in the experimental part, this step allows a sulfonated product equal to the reaction product, to be recovered.
The following examples are provided for a better illustration of the present invention.
The comparative examples show how a simple treatment consisting of (a) dilution of the panel with hexane in the presence of another filter aid, (b) filtration of the suspension thus obtained, is not sufficient.